1. Field of the Invention:
This invention relates to bipolar circuits and more particularly to a bipolar polarity guard circuit.
2. Prior Art:
Electronic instruments and circuits exist that require the application of an external power source. In many instances, the instrument or circuit may be damaged if the polarity of the voltage from the external power source is applied in the reverse direction. For example, the circuitry in a telephone using electronics such as touch tone generators, tone ringers, or speech networks, require a unipolar voltage supply. The twisted wires supplying the voltage to the telephone may be improperly connected, thereby failing to supply the required polarity. In order to prevent damage caused by an improper polarity, many polarity guard circuits have been designed that present a positive polarity on the output terminals. The well-known diode bridge accomplishes this objective, but with a comparative high resistance and associated high voltage drop. The circuit disclosed in WIRELESS WORLD, Volume 84, page 1507 (March 1978) and Measurement Techniques, Volume 18, No. 6, page 873 (June 1975) illustrates how four bipolar transistors may be arranged in bridge fashion to insure a positive voltage output. Two PNP transistors and two NPN transistors, forming two current paths, have their bases coupled to the input terminals to sense the input polarity, thereby determining which of the paths will apply the desired output polarity. U.S. Pat. No. 4,034,168 uses four PNP transistors, along with several resistors, capacitors and coils for achieving the desired output polarity.
The bipolar circuits of the prior art are inefficient in that an additional current path is formed through the transistors that are biased off. All bipolar transistors have internal diodes from collector to base and emitter to base. Normally, these internal diodes are reverse biased and present no problem. However, for the transistors of the prior art bridges, that are biased off, the collector to base diodes are forward biased and allow current to pass back to the source. This collector to base current path reduces the efficiency of the circuit.
Thus the need exists for a more efficient bipolar polarity guard circuit that prevents current from leaking through any transistor that is biased off.